Due to the tremendous success of the Internet, it has become a challenging task to make use of the Internet Protocol (IP) over all kinds of links. However, because of the fact that the headers of the IP protocols are rather large, it is not always a simple task to make this come true for narrow band links, for example cellular links. As an example, consider ordinary speech data transported by the protocols (IP, UDP, RTP) used for Voice-over-IP (VoIP), where the header may represent about 70% of the packet resulting in a very inefficient usage of the link.
The term header compression (HC) comprises the art of minimizing the necessary bandwidth for information carried in headers on a per-hop basis over point-to-point links. The techniques in general have a more than ten-year-old history within the Internet community; several commonly used protocols exist such as RFC 1144, RFC 2507 and RFC 2508. Header compression takes advantage of the fact that some fields in the headers are not changing within a flow, or change with small and/or predictable values. Header compression schemes make use of these characteristics and send static information only initially, while changing fields are sent with their absolute values or as differences from packet to packet. Completely random information has to be sent without any compression at all.
Header compression is thus an important component to make IP services over wireless, such as voice and video services, economically feasible. Header compression solutions have been developed by the Robust Header Compression (ROHC) Working Group of the IETF to improve the efficiency of such services.
When header compression is used over reordering links, such as IP tunnels or other multi-hop virtual circuits, the reordering of packets can generally impact a header compression algorithm in at least three different ways:                1) the compressor and decompressor contexts can get out of synchronization;        2) packets can be erroneously decompressed by the decompressor (detected);        3) packets can be erroneously decompressed by the decompressor (undetected).        
One problem addressed by invention is that the secure reference principle is not robust when reordering of packets occurs between compressor and decompressor. In particular, the usage of the secure reference principle over links that can reorder packets may lead to packets being erroneously decompressed and then forwarded to upper layers (point 3 above).
Note that this does not occur with the optimistic approach because redundant information (e.g. a checksum) to prevent decompression errors is used in all packets, and faulty decompression due to reordering can thus be detected by the decompressor. Erroneous packets can then be discarded instead of being forwarded to upper layers.